The present invention relates to a progressive-power spectacle lens to correct eyesight, and particularly, to a progressive-power lens having a prismatic power to correct heterophoria of an eye.
A progressive-power spectacle lens has a front surface (an object side) and a back surface (an eye side), and at least one of the surfaces is configured as a progressive-power surface whose dioptric power varies depending on the position thereon. FIG. 59 is a plan view showing one example of a progressive-power surface 2 of a conventional progressive-power lens 1. The progressive-power lens 1 includes a distance portion 3 having a dioptric power for distance vision at an upper area of the lens, a near portion 4 having a dioptric power for near vision at a lower area of the lens, and an intermediate portion 5 having a progressive dioptric power changing between the distance portion 3 and the near portion 4. A fitting point E that is a reference point with respect to a position of a user""s eye when the lens is installed in a frame, a distance reference point F, a near reference point N and a prism reference point PR for measuring dioptric powers are defined on the progressive-power surface 2.
Since the curvature of the progressive-power surface 2 gradually increases or decreases from the upper side to the lower side within the intermediate portion, the thickness of the upper edge is different from that of the lower edge when the front and back surfaces are arranged to be perpendicular to a common normal at the center thereof.
FIG. 60A is a cross sectional side view of a progressive-power lens that is designed such that the front and back surfaces are perpendicular to a common axis at the center thereof. In this example, the lens of FIG. 60A has a plus distance dioptric power and the front surface thereof is formed as a progressive-power surface. As shown in FIG. 60A, the entire lens becomes exceedingly thick to keep a necessary thickness at the lower edge, which increases the weight of the lens. In order to reduce the thickness and weight of the lens, a method known as xe2x80x9cprism thinningxe2x80x9d for relatively tilting the front and back surfaces to uniform the thickness at both upper and lower edges as shown in FIG. 60B is generally employed. This method introduces unprescribed prism effect in the lens.
FIG. 61A is across sectional view of another example of a conventional progressive-power spectacle lens, having a minus distance dioptric power. In this example, a front surface is formed as a progressive-power surface that is designed such that the front and back surfaces are perpendicular to a common axis at the center thereof. As shown in FIG. 61A, the thickness of the upper edge and the lower edge are unbalanced. Application of the prism thinning to the lens balances the thickness as shown in FIG. 61B, however, an unprescribed prism effect is introduced. It should be noted that wedge marks indicated in the lenses of FIGS. 60B and 61B show the unprescribed prism effect introduced by the prism thinning and the like.
Conventional progressive-power spectacle lenses are designed to include the unprescribed prism effect in order to reduce the thickness/weight and/or improve appearance. Specifically, the conventional progressive-power spectacle lenses are designed such that aberrations are well reduced with the above-described prism effect being introduced. An example of such a conventional progressive-power spectacle lens will be described.
The exemplary conventional progressive-power spectacle lens is designed for a right eye and has a progressive-power front surface and a spherical back surface. The spherical dioptric power is 0.00 diopter (referred to as xe2x80x9cDxe2x80x9d hereinafter), the addition power is 2.00D, the center thickness is 2.53 mm, the outer diameter is 80 mm and the refractive index is 1.60. The unprescribed prismatic effect, whose prismatic power is 1.47 prism-diopter (referred to as xcex94 hereinafter) and whose prism base setting is 270xc2x0, is introduced to thin the lens and to uniform the edge thickness.
FIGS. 62A and 62B show a coordinate system for illustrating performance of the progressive-power surface. The coordinate system is a left-hand orthogonal x-y-z coordinate system. The z-axis is a normal to the progressive-power surface at the prism reference point PR that is the origin of the coordinate system. The y-axis is perpendicular to the z-axis and is a vertical axis when the lens is installed in a frame. The x-axis is perpendicular to both the z-axis and the y-axis and is a horizontal axis when the lens is installed in a frame. A curvature at a point at a distance h (unit: mm) from the z-axis on an intersection line of the progressive-power surface and a plane that includes the z-axis and forms an angle xcex8 (unit: degrees) with the x-axis is expressed as a function C (h, xcex8) (unit: D). A surface power D(h, xcex8) (unit: D) at the point (h, xcex8) is defined by a function D(h, xcex8)=(nxe2x80x2xe2x88x92n)C(h, xcex8). Reference n denotes a refractive index of medium on an object side with respect to the progressive-power surface, and nxe2x80x2 is a refractive index of medium on an eye side with respect to the progressive-power surface.
Table shown in FIG. 63 indicates distribution of the surface power D(h, xcex8) of the progressive-power surface of the conventional lens at a point indicated by the polar coordinate (h, xcex8), i.e., at a point indicated by a distance h (mm) from the prism reference point PR and an angle xcex8 (degree) with respect to the x-axis. Further, FIG. 64 is a graph showing relationships between the surface powers D(h, xcex8) and the angle xcex8 for the distances h=10 mm, 15 mm, 20 mm and 25 mm, respectively. The surface power is relatively low in the distance portion within 30xe2x89xa6xcex8xe2x89xa6150 and relatively high in the near portion within 240 xe2x89xa6xcex8xe2x89xa6300.
FIGS. 65 and 66 are three-dimension graphs showing transmitting optical performances of the conventional progressive-power spectacle lens. FIG. 65 shows a mean refractive power error and FIG. 66 shows astigmatism. In the graphs, plane coordinates represent angles of visual axis (unit: degree) in the vertical and horizontal directions, respectively, and the vertical axis of the graphs represents amount of aberration (unit: D).
A progressive-power spectacle lens for correcting heterophoria (Symptom: visual axes are deviated during a resting period) requires a prismatic effect for correcting heterophoria based on a prescription in addition to the unprescribed prismatic effect introduced by the prism thinning. FIG. 67 is a horizontal cross sectional view of a lens that is designed by adding the prescribed prismatic effect for correcting heterophoria to the above-described conventional progressive-power spectacle lens. The front and back surfaces of the spectacle lens are relatively tilted to provide a necessary prism effect.
The above-described conventional progressive-power spectacle lens is designed such that front and back surfaces-originally designed for a lens having no prescribed prismatic effect are tilted with respect to each other to produce the desired prismatic effect. Therefore, although heterophoria can be corrected, aberration caused by the prescribed prismatic effect is not taken into consideration.
For instance, when the prismatic effect for correcting heterophoria whose prismatic power is 3.00 xcex94 and prism base setting is 180xc2x0 is introduced to the conventional progressive-power spectacle lens, the mean refractive power error and the astigmatism vary as shown in FIGS. 68 and 69, respectively. The mean refractive power error increases at the ear side in the distance portion, the astigmatism increases at the upper portion of the ear side and the nose side in the distance portion, and the balance of the aberrations between the ear side and the nose side is lost across the entire area.
The present invention is advantageous in that there is provided a progressive-power spectacle lens with a prescribed prismatic effect to correct heterophoria of an eye, which has a sufficient optical performance that is equivalent to that of a progressive-power spectacle lens without a prescribed prismatic effect.
According to an aspect of the invention, aberration caused due to a prescribed prismatic effect is corrected by employing a progressive-power surface that is different from that of a reference lens. The reference lens is defined as a hypothetical lens which functions similar to the actual spectacle lens except that a prescribed prismatic effect is eliminated. That is, the reference lens is identical in terms of a spherical power, an astigmatic power, a cylinder axis and an addition power to the actual spectacle lens.
Specifically, condition: xcex94D(h, xcex8) less than 0 is satisfied at a point within a region that is defined by 10xe2x89xa6hxe2x89xa620 and xcex94Bxe2x88x9245xe2x89xa6xcex8xe2x89xa6xcex94B+45, and condition: xcex94D(h, xcex8) greater than 0 is satisfied at a point within a region that is defined by 10xe2x89xa6hxe2x89xa620 and xcex94B+135xe2x89xa6xcex8xe2x89xa6xcex94B+225,
where,             Δ      ⁢              xe2x80x83            ⁢      B        =                  tan                  -          1                    ⁢                                                                  P                1                            ·              sin                        ⁢                          xe2x80x83                        ⁢                          (                              B                1                            )                                -                                    P              0                        ·                          sin              ⁡                              (                                  B                  0                                )                                                                                                        P                1                            ·              cos                        ⁢                          xe2x80x83                        ⁢                          (                              B                1                            )                                -                                    P              0                        ·                          cos              ⁡                              (                                  B                  0                                )                                                          ,xe2x80x83xcex94D(h, xcex8)=D1(h, xcex8)xe2x88x92D0(h, xcex8),
parameters with suffix xe2x80x9c1xe2x80x9d represent values for the spectacle lens,
parameters with suffix xe2x80x9c0xe2x80x9d represent values for a reference lens that is defined as a lens identical to the spectacle lens except that the prescribed prismatic effect is eliminated, the progressive surface of the spectacle lens being configured to compensate for aberration that is caused if the prescribed prismatic effect is added in the reference lens
P is a prismatic power (unit: prism diopter) at a prism reference point, the prism reference point being an origin of an x-y-z orthogonal left-hand coordinate system, z-axis being a normal to the progressive-power surface at a prism reference point that is the origin of the x-y-z coordinate system, y-axis being perpendicular to the z-axis and being a vertical axis when the spectacle lens is installed in a frame, x-axis being perpendicular to both the z-axis and y-axis in the left-hand coordinate system, the x-axis being a horizontal axis when the actual lens is Installed in the frame;
B is a prism base setting (unit: degree),
D(h, xcex8) is a surface power (unit: diopter) of the progressive-power surface of the spectacle lens, the surface power being defined by formula: D(h, xcex8)=(nxe2x80x3xe2x88x92n)C(h, xcex8),
C(h, xcex8) is a curvature (unit: diopter) at a point at a distance h from the z-axis on a line where the progressive-power surface intersects with a plane that includes the z-axis and forms an angle xcex8 (unit: degrees) with x-axis, the z-axis being a normal, at the prism reference point, to the progressive-power surface of the spectacle lens,
n is a refractive index of a medium on an object side with respect to the progressive-power surface of the spectacle lens, and
nxe2x80x2 is a refractive index of a medium on an eye side with respect to the progressive-power surface of the spectacle lens.
Further, in order to respond to various combinations of a spherical power, a cylindrical power, a cylindrical axis direction, a prismatic power and a prism base setting, it is preferable that semi-finished lenses whose front surfaces are finished are stockpiled and a back surface of a selected semi-finished lens is processed according to the customer""s specification.
Optionally, the back surface may be formed as a progressive-power surface. With such a configuration, it is easy to produce the semifinished lenses. Further optionally, the front surface may be formed as a spherical surface.